Optically-pumped semiconductor lasers are now extensively used as compact sources of high-quality continuous-wave (CW) laser-radiation for flow-cytometry and other bio-instrumentation applications. An OPS laser employs as a gain-medium a multilayer structure of semiconductor gain-providing layers (“quantum well layers”) separated by semiconductor spacer layers. An advantageous feature of OPS lasers is that an arbitrary fundamental operating wavelength can be coarsely selected using a particular semiconductor composition for the quantum well layers. Such an OPS gain-structure is typically energized by radiation provided by one or more diode-lasers.
An OPS gain-element (“OPS chip”) includes an OPS gain-structure attached to a distributed Bragg reflector (DBR), which has repeating pairs of quarter-wavelength thick layers of semiconductor material with contrasting refractive indices. The DBR is a cavity mirror in the OPS laser. A detailed description of OPS lasers including intra-cavity frequency converted OPS lasers is provided in U.S. Pat. No. 5,991,318 and in U.S. Pat. No. 6,097,742, both assigned to the assignee of the present invention, and the complete disclosure of each of which is hereby incorporated herein by reference.
In theory, at least, there are semiconductor compositions that could be used as quantum-well layers to generate fundamental laser-radiation at any wavelength in the electromagnetic spectrum between the ultraviolet and the infrared. OPS lasers generate laser-radiation most efficiently and conveniently at near-infrared wavelengths, between about 800 nanometers (nm) and 1100 nm, using quantum-well layers in the gallium indium arsenide phosphide (Ga/In/As/P) system. Because of the high efficiency and corresponding high gain of OPS lasers that employ this system, shorter wavelength laser-radiation can be generated through intra-cavity frequency conversion of the fundamental laser-radiation. Intra-cavity frequency conversion includes frequency-doubling and sum-frequency-mixing operations.
In bio-instrumentation applications, there is a growing demand for compact sources providing CW laser-radiation at ultraviolet wavelengths less than about 400 nm. These are most efficiently generated by sum-frequency mixing (third-harmonic generation) using two intra-cavity optically nonlinear crystals. One of the crystals is arranged for frequency doubling to generate second-harmonic (2H) radiation from the fundamental laser radiation, the other crystal is arranged for sum-frequency mixing to generate third-harmonic (3H) radiation from the second-harmonic radiation and the residual fundamental radiation. By way of example, a fundamental wavelength of 1064 nm can be frequency-doubled to provide second-harmonic radiation having a wavelength of 532 nm. Sum-frequency mixing the fundamental and second-harmonic radiations generates third-harmonic radiation having a wavelength of about 355 nm.
A convenient arrangement for generating third-harmonic radiation is to employ type-I frequency-doubling followed by type-II sum-frequency mixing. In type-I frequency doubling, fundamental radiation is plane-polarized in a first polarization-orientation. The second-harmonic radiation is generated plane-polarized in a second polarization-orientation that is orthogonal to the first polarization-orientation. This arrangement provides that the fundamental radiation and second-harmonic radiation automatically have the relative orientations required for type-II sum-frequency mixing. An OPS gain-structure is not polarization selective, so an optical element must be provided in an OPS laser to cause the fundamental radiation to be plane-polarized in the first polarization-orientation. The polarizing element is typically a birefringent filter arranged at its Brewster angle. The birefringent filter also selects a fundamental wavelength from a relatively broad (about 30 nm) gain bandwidth of the OPS gain-structure.
A problem with the above described third-harmonic radiation generating arrangement is that the efficiency of type-II sum-frequency mixing falls off steeply at wavelengths less than 355 nm. This can be overcome by using type-I sum-frequency mixing, which can generate third-harmonic radiation with practical efficiencies for wavelengths as low as 280 nm. Unfortunately, in an arrangement wherein both frequency-doubling and sum-frequency mixing are type-I operations, an additional birefringent optical element is necessary between the second-harmonic generating and third-harmonic generating crystals to bring the fundamental radiation and second-harmonic radiation into the same first polarization-orientation.
In many of the above-discussed bio-instrumentation applications, it is usual to have several radiation sources, providing radiation at a different wavelengths. The cost of the radiation sources can be a critical issue. Birefringent optical elements contribute significantly to the cost of OPS laser-radiation sources. In an intra-cavity frequency-tripled OPS laser, there must be at least three birefringent elements, two of which are the optically nonlinear crystals used for the frequency-conversion. It would be advantageous to be able to perform the above-described type-I third-harmonic generation without need for the additional birefringent element.